Multimedia projection systems have become popular for purposes such as conducting sales demonstrations, business meetings, classroom training, and for use in home theaters. In typical operation, multimedia projection systems receive analog video signals from a video unit and convert the video signals to digital information to control one or more digitally driven light valves. Depending on the cost, brightness, and image quality goals of the particular projection systems, the light valves may be of various sizes and resolutions, be transmissive or reflective, and be employed in single or multiple light path configurations. (Hereinafter, projection systems may also be referred to as “projectors.”)
Optimized sets of multimedia projector characteristics have been achieved by employing reflective light valves, the most common types of these reflective light valve optical arrangements are deflected mirror arrays and reflective liquid crystal light valves. Deflected mirror arrays are very efficient reflectors that do not require polarizers for operation. However, they are quite expensive, require off-axis illumination, and often employ unusual optical elements, such as specialized prisms, to compensate for the off-axis light path angles generated.
Reflective liquid crystal light valves are typically fabricated on a silicon substrate and are, therefore, referred to as liquid crystal on silicon (LCOS) light valves. They are much less expensive than reflected mirror devices, but require specialized polarizers for operation, which results in significant light transmission losses.
LCOS light valve-based projector architectures employ linear polarized light-sensitive devices for receiving light from a randomly polarized light source, reflecting the light off the light valves, and redirecting the reflected light, depending on its polarization direction or state, either out through a projection lens or back toward the light source. The polarization state of the light is determined by an electronic image pattern applied to the light valve.
There are several different optical architectures for employing LCOS light valves. One variation is a multipath optical architecture that provides a separate path for each of the primary color (red, blue and green) lights. The different color lights are routed through a series of polarization beam splitters, filters, and wave plates to a color specific reflective LCOS light valve. Polychromatic light is optically divided to provide each of the three pathways with its associated color light. A light valve, which is provided in each pathway, is modulated with its respective color data. The individual pathways are then recombined into a converged projected color image. Another variation is a single-path multimedia projector that typically includes a color wheel-based frame-sequential color (FSC) optical arrangement. In this arrangement, polychromatic light rays emitted by a light source are directed through the color filter segments of the color wheel. The resulting FSC light travels along a single light path that color timeshares a single light valve.
The multipath optical architecture generally provides an increased image brightness compared to the single-path architecture. Image brightness is also a function of the amount of collected light from the lamp and the color efficiency, which is generally lower for the single-path architecture. Nevertheless, the single-path architecture is generally preferred because the resulting systems tend to be lighter weight, lower cost, and more compact in size. All of these factors can be further improved if the light produced by the lamp (light source) can be collected efficiently and propagated through the optical components optimized for a low étendue, which enables using reduced-size optical components.
FIGS. 1–6 illustrate these problems in further detail. In particular, FIG. 1 shows a prior art example of a conventional light source 100 used in conjunction with a single-path architecture. The light source includes an arc lamp 101 mounted at a focus of an elliptical reflector 102. Polychromatic light rays emitted by arc lamp 101 are converged by elliptical reflector 102 to propagate along optical axis 106 through color filter segments of color wheel 103 and light integrator 104. Color wheel 103 preferably includes R, G, B and light-purplish filter segments. Because the light from arc lamp 101 is typically greenish (deficient in red), the light-purplish (non-white) filter segment produces a more accurate white color point and overall color gamut for multimedia projector. Alternatively, the color wheel 103 could be replaced with other types of color modulators, such as a liquid crystal-based color switcher. After the FSC light passes through the color wheel 103, it passes through a light integrator before it enters the remaining components of an image projection system 105.
One of the functional purposes of an illumination system is to output a large amount of light energy. However, the emitted light energy is restricted by constraints on the physical dimensions of the light source as well as the amount of light acceptable by downstream optical components. The amount of light that is acceptable to an optical component is a function of its area and the light flux throughput, or étendue. The geometric entity, étendue E, is defined as the product of the transverse sectional area of a light beam and the divergence angle of the beam. Étendue is also referred to as geometric extent.
Referring to FIG. 2, étendue E is a geometric entity that is represented mathematically by Eq. 1:
                              E          =                                    ∫                              ∫                                                      cos                    ⁡                                          (                      ϕ                      )                                                        ⁢                                      ⅆ                    A                                    ⁢                                      ⅆ                    Ω                                                                        =                                          A                ⁢                                                                  ⁢                Ω                            =                                                A                  ⁢                                                                          ⁢                  π                  ⁢                                                                          ⁢                                                            sin                      2                                        ⁡                                          (                      θ                      )                                                                      =                                                      A                    ⁢                                                                                  ⁢                    π                                                        4                    ⁢                                                                  (                                                  f                          /                          #                                                )                                            2                                                                                                          ,                            Eq        .                                  ⁢        1            where Ω defines a cone of light 201 diverging through a cross-sectional area A. f/# is a measure of the relative aperture of a lens, the square of which measures the area of the light gathering capacity.
Étendue is important because in an optical system it cannot be reduced without a corresponding reduction in light flux. It is of particular importance in the efficient collection of light flux from a light source, such as light source 100, which effectively establishes the lower limit of étendue for the entire optical system.
FIG. 3 shows a typical reflector 220, such as a parabolic or elliptical reflector (similar to the one used in FIG. 1) which is a major component establishing the intensity of light rays exiting a light source. A numerical aperture NA of the light exiting reflector 220 depends on an angle θ from which the light rays exit reflector 220 relative to an optical axis 222. As shown below in Eqs. 2, 3, and 4, the theoretical maximum numerical aperture occurs at about θ=60°.4f(f−r cos(θ))=r2 sin2(θ),  Eq. 2
                              r          =                                    2              ⁢              f                                      1              +                              cos                ⁡                                  (                  θ                  )                                                                    ,        and                            Eq        .                                  ⁢        3                                                      NA            ⁡                          (              θ              )                                =                                                    sin                ⁡                                  (                  δ                  )                                            ≈                                                                    1                    /                    2                                    ⁢                                                                          ⁢                  a                  ⁢                                                                          ⁢                                      sin                    ⁡                                          (                      θ                      )                                                                      r                                      =                                          a                                  4                  ⁢                                                                          ⁢                  f                                            ⁢                              sin                ⁡                                  (                  θ                  )                                            ⁢                              (                                  1                  +                                      cos                    ⁡                                          (                      θ                      )                                                                      )                                                    ,                            Eq        .                                  ⁢        4            where f is the reflector focal length, a is the length of an arc lamp arc, and δ is the ray divergence angle resulting from arc length a.
Étendue is also impacted by light-integrating components such as light tunnels and lenslet arrays that are typically employed to improve the uniformity of light rays exiting light sources. FIG. 4 shows a typical lenslet array light integrator 230 having first and second integrator plates 232 and 234. The limiting étendue E of light integrator 230 may be represented by Eq. 5:
                              E          =                                    A              ⁢                                                          ⁢              Ω                        =                                                            A                  lens1                                ⁢                                  A                  int2                                                            d                2                                                    ,                            Eq        .                                  ⁢        5            where A is the aperture area, Ω is the solid angle of the transmitted beam, Alens1 is the area of the first integrator plate 232, Aint2 is the area of the second integrator plate, and d is the distance between the two plates.
Assume, for example, a typical light source employing arc length a and reflector 220 of FIG. 3 combined with light integrator 230 of FIG. 4. FIG. 5 shows an array of arc images 240 projected from reflector 220, through first plate 232, and onto the lenslets of second plate 234 of light integrator 230. Note that the sizes of arc images 240 vary in area as a function of their reflection positions off reflector 220. Arc images 240 near the center of second plate 234 are relatively small, increase rapidly in area towards regions radially removed from the center, and dwindle in size near the periphery of second plate 234. Accordingly, FIG. 5 demonstrates that the system étendue is not efficiently (geometrically) filled with light.
FIG. 6 shows a graph of light intensity as a function of reflection angle θ for an elliptical reflector, which explains why the sizes (areas) of arc images 240 vary. In particular, because of arc lamp shadowing, almost no light is reflected between zero and three degree reflection angles, which results in the “hole in the middle” effect seen in this type of arc lamp and reflector combination. Light intensity double-peaks between 5 and 12 degrees and gradually diminishes out to 30 degrees. This effect becomes more pronounced as the étendue of a projector is decreased.
What is still needed, therefore, is an illumination system which achieves a suitable light collecting efficiency at a small étendue. More particularly, what is needed is a reflector based illumination system in which the arc image remains substantially constant in area as a function of reflection angle θ. Such an illumination system would be advantageous in designing a compact, lightweight, and/or low-profile multimedia projection system that achieves a bright and/or high-quality projected image at preferably a relatively low cost.